Methods and devices for low-temperature separation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik [Low-temperature technology], 2nd edition 1985, Chapter 4 (pages 281 to 337).
The distillation column system of the invention comprises a two-column system (for example a classical Linde double-column system) for separating nitrogen/oxygen having a high-pressure column and a low-pressure column which are in a heat-exchange relationship with one another. The heat-exchange relationship between high-pressure column and low-pressure column is generally effected by a main condenser in which overhead gas of the high-pressure column is liquefied against vaporizing bottom liquid of the low-pressure column. In addition to the columns for separating nitrogen/oxygen, the distillation column system can comprise other devices, for example for producing other air components, in particular noble gases, for example an argon production stage which comprises at least one crude argon column, or a krypton-xenon production stage. The distillation column system, in addition to the distillation columns, also comprises the heat exchangers directly assigned thereto, which heat exchangers are generally constructed as condenser-evaporators.
The majority of modern air separation plants are constructed on the basis of what is termed a double column. This system of two coupled columns having differing working pressures enables not only the production of gaseous oxygen-, argon- and nitrogen-containing products, but also liquid fractions. These liquids can be taken off from the air separation plant as liquid end products or are internally compressed (brought to the higher pressure in a pump and warmed), so they are then available as gaseous pressurized products.
If such liquid fractions are taken off from the double-column system, a corresponding amount of air must be preliquefied before being fed into the double column, that is to say some of the air is passed into the double-column system in the gaseous state (feed air to the high-pressure column and, e.g., air from the Lachmann turbine, which is fed directly into the low-pressure column) and some of the air is fed into the double-column system in the liquid state (throttling stream and liquid air from Claude turbine, where present). If many products are taken off in the liquid state, the proportion of preliquefied air increases correspondingly.
Since only the lower sections of both columns are charged with liquid air, the preliquefied air shares only few of the rectification processes in the double column (compared with gaseous air). Therefore, the preliquefaction has an adverse effect on the rectification processes in the double column. With increasing air preliquefaction, the oxygen yield decreases (and also the argon yield, if the system produces argon). The efficiency and economic efficiency of the air separation plant decrease.
In order to intensify the rectification (in particular in the upper sections of both columns), resort is made to measures such as what is termed a “feed compressor” (which compresses some of the product from the upper part of the low-pressure column to the pressure of the high-pressure column and this is fed into the high-pressure column) and/or attempts are made to use what is termed a nitrogen cycle for generating cold (the air in this case is not liquefied upstream of the double column but within the pressure column by liquid nitrogen). These measures, however, mean a higher energy consumption and make the overall plant more expensive as a result of a higher number of heat exchangers and/or machinery.